Russian-installed authorities in Ukraine told all residents of the city of Kherson to leave “immediately” Saturday ahead of an expected advance by Ukrainian troops waging a counteroffensive to recapture one of the first urban areas Russia took after invading the country.

In a post on the Telegram messaging service, the pro-Kremlin regional administration called on civilians to use boat crossings over a major river to move deeper into Russian-held territory, citing a tense situation on the front and the threat of shelling and alleged “terror attacks” by Kyiv.

Kherson has been in Russian hands since the early days of the nearly 8-month-long war in Ukraine. The city is the capital of a region of the same name, one of four that Russian President Vladimir Putin illegally annexed last month and put under Russian martial law on Thursday.

On Friday, Ukrainian forces bombarded Russian positions across the province, targeting pro-Kremlin forces’ resupply routes across the Dnieper River and inching closer to making a full assault on Kherson city. Ukraine has retaken some villages in the region’s north since launching its counteroffensive in late August.

Russian-installed officials were reported as trying desperately to turn Kherson city — a prime objective for both sides because of its key industries and ports — into a fortress while attempting to relocate tens of thousands of residents.